1. Field of the Invention
This invention is directed to solid electrolytes containing a polymer matrix and (optionally) an electrolyte solvent (plasticizer) for the polymer matrix. In particular, this invention is directed to solid electrolytes containing an ion salt having an organometallic anion (hereinafter referred to as an organometallic ion salt). The organometallic ion salt can partially or completely replace the inorganic ion salt and electrolytic solvent heretofore added to the electrolyte in prior art electrolyte compositions.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing a polymer matrix, an optional solvent and an organometallic ion salt incorporated into the polymer matrix.
This invention is also directed to a method for enhancing the ion conduction of a solid electrolytic cell by employing a solid electrolyte which contains the organometallic ion salt of the invention.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating an inorganic ion salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
Solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable inorganic ion salt as a separate component. The inorganic matrix may be non-polymeric [e.g, .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte incorporating a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and an organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
Regardless of which of the above techniques is used, the resulting solid electrolyte could be improved with respect to conductivity. For example, even under the best of circumstances, the inorganic ion salts typically have a transference number between 0.4 and 0.55, meaning that the ion salt carries only between 40% and 55% of the total plus (+) charge.
The relatively low transference number also adversely affects cumulative capacity. The cumulative capacity of a solid battery is defined as the summation of the capacity of the battery over each cycle (charge and discharge) in a specified cycle life.
Another area in need of improvement is solvent characteristics, in particular solvent volatility. A significant quantity of solvent evaporates during processing to form the solid electrolyte. To retain sufficient solvent in the final product therefore requires a higher initial amount of solvent in the electrolyte formulation. This increases both raw material costs and processing costs since the evaporated solvent must either be recaptured and recycled or disposed of.
In view of the above, the art is searching for methods to increase the conductivity of solid electrolytes as well as to increase the cumulative capacity of solid batteries employing such electrolytes. Furthermore, there is a need in the art for reducing or eliminating solvent volatility.